The deals from BT, Virgin and Sky lasted 12 months, EE's was 18 months and Talk Talk's was 24 months in length.

Researchers found the BT deal rose by 67%, the Sky deal increased by 53%, the EE deal went up by 36% and the Talk Talk deal rose by 28%.

There was no increase when the Virgin Media deal ended.

On average the bills rose by 43%, or £9.45 more a month, adding £113 a year.

The charity also surveyed more than 3,000 broadband customers and found they stayed on the same contract for an average of four years.

Pensioners were more than twice as likely than customers under 65 to have been on the same contract for more than 10 years.

Similarly those on low incomes were almost three times more likely than high earners to remain in a contract for 10 years or more.

Ms Guy said: "Older customers and those who have less money are more likely to stay with their supplier for longer meaning their loyalty penalty could reach over a thousand pounds.

"The Government has rightly put energy firms on warning for how they treat loyal customers - the actions of broadband firms warrant similar scrutiny. Extra protections for vulnerable

A Department for Culture Media and Sport spokesman said: "Measures we are introducing in the Digital Economy Bill will provide customers with more information and make it easier for them to switch to a better, fairer deal.

"We will shortly be consulting on how to improve things further for consumers, including a requirement for providers to notify them automatically when their fixed contract period ends."